The present invention relates to a low and broad bandwidth shorted microstrip antenna which is shorted at one side thereof and may be mounted on a mobile body in a mobile communication system and provided with improved beam tilting and impedance matching characteristics.
A shorted microwave strip antenna (SMSA) is a half-sized version of an ordinary patch antenna and is characterized by a miniature, light weight and low height costruction. Due to such advantages, an SMSA is suitable for use as an antenna which is mounted on a mobile body in a mobile communication system. Generally, an SMSA includes a grounding conductive sheet on which a feed connector is mounted, a radiating conductive sheet which faces the grounding conductive sheet with the intermediary of air or like dielectric material, and a connecting conductive sheet positioned at the shorted end of those two conductive sheets perpendicular to the surfaces of the latter in order to connect them together.
In the above-described type of SMSA, assume X and Y axes in a general plane of the emitting and the grounding conductive sheets (the Y axis extending along the general plane of the connecting conductive sheet), and a Z axis in the general plane of the connecting conductive sheet which is perpendicular to the X and Y axes. Then, emission occurs in the SMSA due to a wave source which is developed in the vicinity of a particular side of the radiating conductive sheet which is parallel to the Y axis and not shorted. If the size of the grounding conductive sheet is effectively infinite, the SMSA is non-directional in the X-Z plane on condition that Z is greater than zero; if it is finite, the SMSA obtains the maximum directivity in the vicinity of the Z axis. When the radiating conductive sheet is positioned at, for example, substantially the center of the grounding conductive sheet, the directivity is such that the maximum emission direction is tilted from the Z direction, resulting in a decrease in the gain in the Z direction. This is accounted for by the fact that the wave source of the SMSA is not located at the center of the grounding conductive sheet. A prior art implementation to eliminate such beam tilts consists in dimensioning the grounding conductive sheet substantially twice as long as the radiating conductive sheet in the X direction. This kind of scheme, however, prevents the SMSA from being reduced in size noticeably, compared to an ordinary microstrip antenna (MSA). It therefore often occurs that it is difficult for an SMSA to be installed in a mobile body such as an automotive vehicle.
Further, as regards an SMSA having a relatively small connecting conductive sheet, current is allowed to flow into the jacket of a cable which is joined to a feed connector. This would render the impedance matching characteristic of the antenna unstable and disturb the directivity.